The UvrABC excision endonuclease of E. coli is an extraordinary enzyme that recognizes bulky lesions in damaged DNA and cuts the damaged strand on both sides of the lesion. The objective of this research is to characterize the UvrABC nuclease and its reacations in an effort to understand the roles of the 3 uvr proteins in the overall UvrABC reaction and to determine the specific features of damaged DNA that are recognized by the enzyme. Detailed studies of the enzyme with various substrates will be done using standard enzymological methods. In experiments designed to locate the positions on DNA that interact with the UvrABC enzyme, several substrates with a single lesion at a defined site have already been constructed. These lesions include carbodiimide, AAAF, psoralen monoadducts and psoralen crosslinks. Other experiments will be done to determine the role of the individual subunits and of ATP in the overall reaction. While this enzyme is intrinsically interesting, the UvrABC excision nuclease may well turn out to be similar to multigenic excision repair enzymes found in yeast and humans, particularly since defects in such enzymes are known to cause a genetic predisposition to cancer.